Compound ferroelectric-ferroelastic crystal

ABSTRACT

A COMPOUND FERROELECTRIC-FERROELASTIC CRYSTAL DEFINED BY THE EXPRESSION   (RXR&#39;&#39;1-X)2O3-3MO1-EWEO3   WHERE R AND R&#39;&#39; ARE AT LEAST ONE RARE EARTH ELEMENT, X=0-1.0 AND E=0-0.2 AND HAVING THE GADOLINUM MOLYBDATE CRYSTRAL STRUCTURE BELONGING TO THE ORTHORHOMBIC SYSTEM AND THE POINT GROUP MM2. THE CRYSTAL HAS UNEQUAL A AND B LATTICE CONSTANTS, WHEREIN B-A/2A IS MORE THAN 1.0X10**-3, THUS EXHIBITING FERROELECTRIC-FERROELASTIC PROPERTIES.

KEncHlRo Alzu ETAL 3,773,89@ COMPOUND FRROELECTRIC-FEHROELASTIC CRYSTAL Nov. 20, 1973 4 Sheets-Sheet 2 .Filed Aug. 25, 1971 F/G. 5a

INVENTORS Kaucmno Axzu,AKo KuHADAIHmown Yumoo' Slmcm AsHznAl Yosm FURUHATA BY C? Ovu.

Quid mi@ 4 ATTORNEYS COM-POUND FERROELECTRICFERROELASTC CRYSTAL .Filed Aug. 25, 1971 4Sheets-Sheet .1

INVENTORS Keucnmo Alz UIAKao Mum/fa13A/HmmmYummo,

.SAKILH ASHiDAYOSHI FURUHATA BY QM@ mmm 1. mi

ATTORNEYS 3,773,898 COMPOUND FERROELECTRIC-FERROELASTIC CRYSTAL Keiichiro Aizu, Tokyo, Akio Kumada, Kodairo, Hiroshi Yumoto', Kokubunji, Sakichi Ashida, Fuchu, and Yoshio Furuhata, Kodaira, Japan, assignors to Hitachi,I Ltd., Tokyo, Japan Continuation-impart of abandoned application Ser. No. 38,653, May 22, 1970. This application Aug. 2S, 1971, Ser. No. 174,810

Claims priority, application Japan, Aug. 2, 1967; liii/49,674; Sept. 18, 1967, i2/59,450 Int. Cl. Ctllg 39/00 U.S. Cl. 423-'263 4 Claims ABSTRACT F THE DISCLOSURE A compound ferroelectric-ferroelastic crystal dened by the expression where R and R' are at least one rare vearth element,

' date crystal structure belonging to the orthorhombic system and the point group mm2. The crystal has unequal a and I1 lattice constants, wherein ba/2a is more than 1.0)(-3, thus exhibiting ferroelectric-ferroelastic propertes.

Field of the invention This invention relates to a compound crystal having the ferroelectric-ferroelastic property.

Description of the prior art As is well-known, ferroelectric material has a piezoelectric effect, generating a voltage by an applied mechanical stress or strain or generating a mechanical stress or strain by an applied voltage. Of the materials having the piezoelectric effect lead zircotitanate (hereafter abbreviated as P.Z.T.) has a large piezoelectric effect, which is due to an electric or a mechanical strain induced in the crystal with applied stress or voltage. For example, when a piezoelectric relay is made of P.Z.T. pieces with a length of 20 to 30 mm. and a contact gap of 0.1 mm., the expansion An by the longitudinal effect (i.e. the applied voltage V3 and the generated strain are in the same direction) is given by An=d33v3 where 33 is a constant depending on the material and the direction of voltage. The maximum value of das of P.Z.T. is

d33e600 l0-12 (m/V) So, in order to switch a contact gap of the order of 0.1 mm. the voltage V3 must be When P.Z.T. pieces with n lateral length 1:30 mm. and a longitudinal length nzOJ mm. are employed and their trnnsverc effects (i.e. the applied electric ticld E and the strain X nre perpendicular to each other) are used, in

order to yield a lateral strain of the order of 0.1 mm. the voltage V3 must be V3=1.11 kv. as

Therefore, it is understood that even if the best P.Z.T.

is employed a large driving voltage is required.

After long research on ferroelectric materials it has been found that some e.g. Rochelle salt and potassium dihydrogen phosphate (hereafter abbreviated as KDP) are different from barium titanate in that the application of astress above a certain value can reverse their spontaneous polarization (i.e. the electric polarization possessed by the material in the spontaneous state). Further, it has been found that the crystal is distorted rapidly with this reversal in polarization causing a kind of creep phenomenon. This anomalous distortion disappears when the polarization reversal is finished. Hysteresis loops are found to be observable between the stress and the polarization (X vs PS), and between the stress and the strain (X vs xs). Also, has been found that the mediano-electrical conversion efficiency due to this novel effect is larger than that of the conventional piezoelectric etfect. For example, when the ferroelectric material KDP, Curie point of which is 123 K., is thinly 45 Z cut (i.e. cut in parallel with (011), (O10) and (110) surfaces), to tix an electrode on the Z (Oil) surface of its thin plate, and an ac voltage is applied thereto, a hysteresis is observed between the stress and the voltage in the ferroelectric phase in accordance with the polarization change. If the thin plate has a length l and the variation in length of thin plate due to the "polarization change is AI, then a spontaneous strain xs (i.e. the strain possessed by KDP in the spontaneous state) is given by The electric field Ec required for the polarization change (hereinafter referred to as the coercive field) is given by E=1000 v./cm.

When a voltage V3=50 v. is applied to a KDP piece with :15 mm. and 1220.5 mm., the variation in length becomes AI=0.12 mm. Thus, it is possible to make a piezoelectric relay through theuse of the longitudinal elicct of KDP. (131' in the above relations is the apparent piezo electric modulus. Since 131' is 133 tintes larger than dal of P.Z.'l`.

3 SUMMARY oF THE INVENTION The 'general object of this invention is to provide a compound fcrroelectric-ferroelastic crystal defined by the expression (RxR1x)2O3-3Mo1 eWO3, where l?. and R' are at.least one rare e'arth element, x=0-l.0 and e=0-0.2, with a gadoliniurn molybdate crystal structure (hereinafter referred to as the GMO crystal structure) belonging to a type ZZmFmmZ.

Other objects, features and advantages of the present invention will readily be apparent from the following detailed description of certain preferred embodiments taken in coniuriction with the accompanying drawings.

BRIEF DESCRPTION OF THE DRAWINGS elastic material belonging to the species 42mFmm2.

FIGS. 6(a) and (b) are side and lateral cross-sectional views showing a mechano-electrical coupling` device using a compound crystal made in accordance with the present invention.

PIG. 7 is an explanatory view'of a piezoelectric relay using a erroclectric-ferroelastic material belonging to the 42mFmrn2 species.

HG. 8 shows a bimorph type. piezoelectric vibrator using a ferroelectric-ferroelastie material belonging to the i2mFmm2 species.

FlG. 9 and l() show how the bimorph type vibrator shown in FIG. S operates in different ways of piling thin crystal plates.

FIGS. ll(a) and (b) are longitudinal cross-sectional and plan views of a piezoelectric transformer using a compound crystal made in accordance with the present invention.

DEscRrPrroN oF THE PREFERRED EMoDiMENrs Explanation of this invention will be made hereinafter with reference to these drawings.

A ferroelectric material has more than two different orientations of electric polarization in the absence of an electric field and a mechanical stress, and 'these two different orientations can be subjected to the mutual transition thereof by application of an electric field. lt is to be noted here that any two orientation states are identical or enantiomorphous in regard to crystal structure. The ferroelectric material generally displays a polarization (P) vs. electric field (E) hysteresis loop as shown in FIG. 1.

As well-known, the thermodynamic potential tb of the ferroeleetric material under the conditions that the temperature T, the electric held E (vector) and the stress X (second rank tensor) are constant is given by mtln. By the application Qt in electric field h 0f :it

suitable value the difference of il/ between two states becomes considerably large due to the third term of the right hand side of Eq. l. This means that the states are unstable. Therefore, we can realize that there is a possibility of transition o two different states by the electric eld.

(b) In the absence of an electric field and stress there are more than two orientation states with different strains, the difference of 1p between two states by the application of a mechanical stress X of a suitable value becomes considerably large due to the fourth term on the right hand side of Eq. l. Therefore we can realize that there is a possibility of transition of these two different states by the. mechanical stress.

Further, the material is called ferroelastic material when it has more than two different orientation states of mechanical strain in the absence of an electric eld and a stress and can perform .transitions between two different orientation states by a mechanical stress. The ferroelastic material displays a strain (x) vs. stress (X) square 6hysteresis loop as shown in FIG. 2. It is to be noted here that any two orientation states are identical or enantiomorphou's in respect of the crystal structure.

The material is called ferroelectric-ferroelastic material in this specification when it possesses the ferroelectric and ferroelastic properties simultaneously. Transitions between two arbitrary states can be made by either an electric field or a stress. In the absence of a stress the ferroelectric-ferroelastic material displays the electric field vs. electric polarization hysteresis as shown in FIG. 1 as well as the electric field vs. mechanical strain hysteresis as shown in FIG. 3. In the absence of an electric field the material displays the stress vs. strain hysteresis as shown in FIG. 2 as well as the stress vs. electric polarization hysteresis as shown in PiG. 4.

Of the crystals varying their orientation states of polarization by an electric field or a stress some of them suffer a mechanical strain in a unit cell either when the spontaneous polarization is positive (called 0 states) or negative (called l state, variation in orientation of electric polarization). A crystal suffering no strain is defined as a regular ferroelectric material while a crystal suffering ay strain is dened as an irregular ferroelectric material, the latter being included in the ferroelectricferroelastic material.

Examples of irregular, ferroelectric materials are potassium dihydrogen phosphate (KDP), and gadolinium molybdate, these belonging to the point group mm2 and are expressed by (RXR1 X)2O3-3Mo1 eWeO3 where R and R' are at least one rare earth element, x=0-l.0 and In the hysteresis loops shown in FIGS. 1 to 4, the curves AB and CD correspond to orientation states ditiering one from the other. Half the difference of polarization in the absence of the electric field and mechanical stress is called spontaneous polarization PS while halt the difference of mechanical strain is called spontaneous strain xs. The curves BB and DD corresponding to transient states of transition and electric field EC and stress XC needed for these transitions are called coercive field and coercive strain respectively.

The ferroelcctric-ferroelastic material generally shows a peculiar phase transition towards the paraelectric-paraelastic phase. When two fcrroclectric-ferroclastic materials are the same in regard to their own point group and a corresponding point group of paruelcctric-paraclnstic phase, they are classified ln the same species There aie 42 species classified in the following Table l, where the symbol of the point group of the ferroclecti'ic-fcrroclastic material is written to the right of the character "F" :ind the symbol of the point group of the corresponding paraelectric-parnelastic phase is written to the lett of "l-i." The first column shows the species, the second column the number of orientation states possessed by the species, and the third column the material represented by the symbol.

the noncubic system) or parallel to one of the face diagonals' of a cubic lattice of the paraelectric-paraelastic phase (belonging to the non-cubic system).

TAB-Ln 1 Num- Correber ot spending states material Num- Correber of sponding states material Species Species l Methylarnmonium aluminum sulfate dodecahydrate and ammonium cadmium sulfate.

The applied electric field or stress and their types and directions causing transitions betweenl two orientation states are determined for each of the 42 species.

In the case of KDP which is an example of a ferroelectric-ferroelastic material belonging to 2mFmrn'2. species, if the z axis is defined parallel to the axis of symmetry of the paraelectric-paraelastic phase and x and y axes perpendicular to two mirror planes of symmetry, the a and b axes of the unit cell of the ferroelectric-ferroelastic material belonging to this species orientate as shown in FIGS. 5(a) and (b) (called 0 and 1 states respectively). The orientation of the spontaneous polarization is parallel with the z axis (Le. perpendicular to the drawing paper), the direction being positive in the 1 state while negative in the 0 state. Transition from the 0" to the l state by an electric field is obtained by applying the field in the positive direction of the z axis. Transition from the 0 to the 1 state by a stress is obtained by applying n pressure to a crystal surface perpendicular to the x axis or a tension to a crystal surface perpendicular to the y axis, or a shear stress along a crystal surface making nn angle with both x and y axes. Transition from thc 1" to the 0" state by an electric field is obtained by applying the field in the negative direction of the z axis. Transition from the l to the "0 state by a stress is obtained by applying a pressure to a crystal surface perpendicular to the y axis, a tension to n crystal surface perpendicular to the x axis, both pressure and tension simultaneously, or a shear stress to a crystal surface making an angle with both x and y ases.

. 6A The GMO crystal structure (RxR'x-x) 203 3 Moi-eweoa (wherein R and R' are at least one rare earth element,

x=0-l.0 and e=0-0.2) belongs to the orthorhombic system ?ba2 and the point group mm2. KDP also belongs to the point group mm2 but has the Curie temperature C. and other practical defects. On the other hand, the GMO with the Curie temperature near C. main- 'tains the property of ferroelectric-ferroclastic material over a wide temperature range and a high conversion efficiency of mechanical to electrical energy. Therefore, the characteristic of GMO is excellent for a mechano-electrical coupling device.

Although this type of crystal has been previously described as monocrystal for example, in U.S. Pat. No. 3,437,432, thc crystal structure of gadolinium molybdate with equal crystal lattice constants a and b disclosed there belongs neither to the point group mm2 nor to irregular ferroelectric materials. It does pot even belong to the ferroelectric-erroelastic materials in a broad sense.

Explanation of having the GMO structure and belonging to the point group mm2 will be made hereinafter.

Results of measurements by means of a triaxis X-ray goniometer and the X-ray diffraction method showed that Gd2(M0O4)3 belonging to the rhombic system with the lattice constants a=10.38i0.005 A.

b=10.426i0.005 A.

c=10.709i0.005 A.

and could be expressed by the space group PbaZ and the point group mm2.

EuztMOOOa, '152(M0O03, DyetMOOs and Smz(M0O4)a were isomorphs of Gd2(MoO.,)3, but the lattice constants measured by the X-ray diffraction method differed in the crystal lattice constants a and h. All of these chemical compounds had the GMO crystal structure.

TABLE 2 Sample a (A.) b (A.) c (A.)

E111 (M900): 10. 377i0. 005 10. 472i0. 005 10. 555:1;0. 005 GdztrNIoOils.--" 10. 3SS;;0. 005 10. 426:5). 005 10. 70010. 005 Dy2(Mo04)a 10. 331;l;0. 005 10. 366:1:0 005 10. 603i0. 005 Smq(i\1004)s 10. 478:1;0. 005 10. 51h- 0. 005 10. SML-t0. 005

The values of (b-a) /2a for said Eu2(MoO4)3,

Gd2(M0O4)3 i Dy2(MoO4)3 and Sm,(MoO4)3 are respectively in the ranges of 0.004 to 0.005, 0.0015 to 0.0023, 0.0012 to 0.002, and 0.0010 to 0.002.

The monocrystals of Gd2(MoO4)3, Sm2(MoO4)3,

measured by an X-ray trinxis goniometer. The measurcment was made on a retlection from the surfaces 200, 400, 600, 600, 10 0 0, and 12 0 O .iud the surfaces 001, 002, 003, 004, 005, A006, 007, 00S, O09, 0 0 l0 and 0 0 1l. Thereafter, a reverse voltage was applied in the direction of the c axis. After confirmation of the reversal of the a and b axes the intensity distributions of retlectcd 'f shown in Table 2. The crystal- 7 light from the surfaces O20, 040, 060, 080, 10 0, and

` 0 12 0 were measured by the CuKd line with an X-ray iris.slit of diversion 1, a Ascattering slot of 1 and a receiving light slit of 0.1 mm. The voltage and the current of the X-ray source were 30 kv. and 10 ma. respectively. The scanning speed of the goniometer was l/4/min. and the radius ofthe Geiger counter was 185 mm.

Further, when the poling condition of the above GMO crystal is released, it becomes difficult to observe the difference between the crystal lattice constants a and b. Furthermore, by using a single GMO crystal treated by poling as above mentioned in the mediano-electrical coupling devices illustrated in FIGS. 6 to 1l, effect of the devices can be further improved.

The composite materials used in these devices are chem- (wherein R and R are at least one rare earth element,

x=01.0 and e=0-0.2) made by these positive ions form similar GMO structures. The dimensions of the crystal with the GMO structure belonging to the orthorhornbic system and the point group mm2 which are used in this invention are a=10.388:4: 0.005 A. b=10.426i0.005 A. c: 1070910005 A.

The spontaneous strain xs is given by -z x5 2a 1.0 10

Table 3 shows the properties of Gd2(MoO4)3 used in this invention.

correspond to the u, b and c axes respectively 1.8ti 5 (C./m.2) (direction of the c axis). 1.5)(10! 3X'i0-12 (uil/newton). GX105 (v.lni.).

1.4)(10 (newton/m).

More. than 101 ohm cm.

Strong.

Spontaneous polarization-.. Spontaneous strain- Morlulus o( elasticity. Coert-ive electric icld Cocnlvc stress. Electric resistivity. Waterproof und. chemicalproot properties. Hillcrest-anco und dellqucs- Null.

cuneo properties.

Further. letting the electric capacitance connected to the crystal itself Co and the dielectric constant of free space e0, the dielectric constant of crystal y becomes When a force is applied to a ferroelectric-fcrroelastic bvdy. the force conversion obeys the relations ot' and i g'xl P F; where E is th electric field applied to the crystal, XB is the spontaneous strain and X is the stress. The energy conversion is represented by the relations of n Plz and where Wale, in the electric energy conversion and Wmech is the mechanical energy conversion.

For the vpurpose of comparison, the force conversion and energy conversion of the conventional piezoelectric crystals are given by where KX is the electric susceptibility under a constant stress in the conditions of E=0 and X=0, d is the piezoelectric modulus, and SE is the elastic compliance under a constant field. l

Next, the manufacturing method of the GMO crystal will be explained byexaniples.

-EXAMPLE l Gd2O3 361.8 g. and M003 431.7 g. were mixed to forni a pellet under a suitable pressure. The Gd2(MoO4)3 disc was put in a platinum Crucible, or put on a platinum plate in an alumina Crucible and heated at 700 C. for a few hours in an electric furnace. The sample was taken out of the furnace, powdered, stirred, and pressurized again. The Gd2(MoO4)3 disc was put in a Crucible and heated at 1000 C. for two to four hours in the electric furnace. The product was measured by the powder X-ray diffraction method and was found to have the GMO crystal structure.

Further, the Gd2(MoO4)3 powder obtained above was put in the platinum crucible and fused at about ll C. A platinum wire of l nimo as a seed crystal was dipped in the Gd3(MoO4)3 solution. The temperature was lowered until the fused solution became solid on the wire. The revolution of platinum wire was 30-60 revolutions/ min. Next the platinum wire was pulled up at a rate of 1.5 to 18 nim/hr. The input to the induction coilfwas increased until the diameter of the crystal became about l mm. ln this case, the input to the furnace was controlled so that the diameter of the crystal became l0 to IS/nnu., the crystal of l mnwp being used as a seed crystal. The pulling speed was also 1.5 lo 1S min/hour. V/hen the "crystal grew about 30 to 79 mmap, it was cut out from the GdgUtioQQa solution. The cut-out crystal was put in an after-heater und the temperature was lowered at a rate of 50 to 100 C./hourl to prevent the occurrence of cracks in the crystal. The product thus obtained was identified as Gd2(lloO4)5 belonging to the orthohombic system and the point group mm2, and eruployed in this invention. As will be appreciated by those skilled in the art, the presence of impurities in crystals can produce differences in the properties of the crystals which have been produced from nominally the same mixture. In the present invention the production of a crystal having unequal a and b lattice constants as compared with known crystals having equal a and b lattice constants, for example, as disclosed in U5. Pat. No. 3,437,- 432 is enabled by ensuring that components used in the orginal mixture are as pure as possible and by omitting to add abinder, for example, parain, which can be a source of additional impurities. A crystal having unequal a and b lattice constants will exhibit ferroelectric-ferroelastic lproperties, whilst a crystal having equal a and b lattice constants will not have these properties.

The U.S. Pat. 3,437,432 claims to the subject of a single crystal having the chemical composition expressed by (RxRil-Q 2O3'3M01-@We03 where R and R' are at least one rare earth element, x is to 1.0 and e is 0 to 0.2. But only the case that the lattice constants a and b thereof are equal is disclosed in the patent. In analyzing the crystal structure of the gadolinium molybdate (GMO) single crystal, same X-ray diffraction techniques are employed in the patent and the present invention. That is, the crystal compound produced by the invention has been determined on a Norelco" (trademark) X-ray dilractometer, such as disclosed in the U.S. patent, in column 3, lines 53 to 44 under the equal accuracy in measurement. That is, the error in measurement of r0.005 A. is equal between the patent and the present invention. Under such equal conditions in measurement, the following differences are found therebetween.

U.S.'Pat. 3,437,432 This invention Thus, both gadolinium molybdate single crystals are coucluded to have different lattice constants.

It is essential that as previously disclosed in the pres- 10 ent specification, the anomalous distortion disappears even it the spontaneous polarization is reversed in case the crystal structure has the lattice constants a=b, and hence, the GMO single crystal having such property can not be used 'as a mechanoelectrical coupling element.

The grounds of the above-mentioned differences in the vlattice. constants of a unit cell along a and b axes are as follows:

(a) In this invention, the monocrystals of M003 of no less than 99.999% in purity and Gd2O3 of no less than 99.9% are employed as the host material of the GMO crystal compound, so that such M003 and Gd203 are hardly subjected to inlluences due to theaddition of impurities such as Al, Se, Ga and W which may cause a danger of reducing the difference between the lattice constants a and b.

(b) The U.S. patent uses parain as a binder as dis-n closed in column 8, lines 19124. It is considered that the dissolution of the parafiin reduces' the difference between the lattice constants a and b due to the addition of impurities.

(c5 AIn the U.S. patent, furthermore, the used pellet is heated once from room temperature to 900 C., so that a full reaction for providing the intended single crystal can not be completed since Gd203 melts down into a fused solution of M003, resulting in a single crystal having a low melting point and having a colored portion therein.

On the other hand, the crystal compound according to this invention, as disclosed in the present specication, is fabricated by the steps comprising at first heating Gd2(MoO4)3 to 700 C., pulverizing it in powdered form and heating again the powder at 1000" C. in a crucible, thus resulting a transparent GMO single crystal having a high melting point of ll C.

EXAMPLES 2-49 The method described in Example l was used for manufacturing monocrystals as shown in Table 4. The amount of reactants as shown in this table were heated just under the melting point to form a solid solution.

Next these chemical compounds were heated. Monocrystals were pulled up from the fused solution as described in Example l.

TABLE 4 Molybdate, Monvcrystal para Rare earth oxide, parc Slm-(Meow 431. s (simo.) 348.7

431. 8 (Ellzog) 352.0 431. 8 (DyaOz) 373.0 863. 6 (TbiOr) 748.8 431. 8 (GdeOa) 180.9 .3 431. 8 (GdzOa) 180.9 76.0 431. 8 (GdzOs) 150.9 (Tb401) 187.2 431. 8 (Gde-Oz) 18.9 (DyzOz) 156.5 431. S (GdzOs) 343.7 (YbcOs) 10.7

l. 3 (G (120:) 343.7 (H0201) 18.9 431. 8 (Gd-10:) 313.7 (LugOa) 19.9 431. S (GdzOa) 343.7 (TrugOs) 19.3 431. 8 (Cultos) 343.7 (Sexos) b* 8 .9 (La-03) 16 3 (Gdenlrmh muova--- .s .9 (Phon) 17.0 (Gtla.s\'n.)g (M0003 S .0 (YgCls) 90.3 (Gda-wLm-m): (M004): .S .0 (Laith) 130.0 tG(ls.to'1do.2el)yo-colz 610063-- S (Dy-03) 74.6 iGl-Mllileoynuoh (310003.. .S (llusOal 70.4 (Gdawsluoao'lbano): tMoOih-. .S (SturOa) 69.7 (Gdo.:o1\tn.:o'l`bn.xo)i (r\1oOt):.- S (EUQOJ) 70.4 (GdmYMLaM). (Moo.), s (La .op 33,5 tGd...attenuation (Mao.n s (nino.) 70.4 Gdr..:Srii.x.rEllonYMla @1004); S (Smith) 31.0 ((id-.Ldaasli (110035 .8 (NtL-Onl 16.3 (Gd.\e1`bt.=Yt.iLam): (Moons .s (Tino.) 39.4 lfd:t.\lm.t\\'o.r0d; u (mmm si' .s S 0 S S S S S 8 S S S S .S

(EriOltLl (YzOJ) 22.5

as GMO.

EMBODIMENT 1 As shown inr FIGS. 6(a) and (b), a 5 mm. cube 1 of 45 z cut KDP (cut parallel to the surfaces (001), (110) and (110) whose (O01) surface was fitted with an electrede 2 was disposed in a hollow cylindrical pressure cell 3 made of pyroferrite. The arrangement was dipped in liquid nitrogen to be maintained at an extremely low temperature. A window 8 was provided in the pyroferrite arrangement to promote a rapid permeation of liquid nitrogen and to tix a contact spring 6 (see FIG. 6(b)) easily. A pressure of the order of 100 kg./crn.2 was applied to the arrangement through the end faces 4 of pressure rod 5. A voltage of the order of 5000 volts could be applied to the electrode 2. As shown by the schematic diagram in FIG. 6(b), a capacitor 7 of the order of 380 pf. was connected in parallel with the KDP cube 1 by way of the contact spring 6, obtaining an electrical energy of 6X 10'3 joule.

As pointed out above a single GMO crystal may be used as alternative to KDP.

EMBODIMENT 2 As shown in FIG. 7, a z Cut KDP element 1 with the dimension of 0.5 mm thickness, 2 mm width and 15 mm length (cut perpendicularly to the c axis) whose (001) surface was litted with electrodes 2 on both sides, another surface 10 being fitted with a relay contact portion 11, was disposed in a low temperature bath using for example liquid nitrogen. When a relay circuit 13 was connected to the contact portion 11 and a voltage V of 50 volts was applied across the electrodes 2 by way of the terminals 14, a polarization change was caused in the KDP element 1. As the KDP crystal 1 expanded in the direction of the contact portion and closed it, a current flowed through the relay circuit 13. When a reverse voltage of -50 volts was applied, the crystal contracted in the direction of the Contact portion so that the relay was cut off. Therefore, the polarization change of KDP by an electric eld could be utilized to make a self-keeping type piezoelectric relay. The driving element for such piezoelectric relay could -be made of ferroclectric-ferroelastic material crystals including not only KDP but also GMO.

EMBODIMENT 3 As shown in FIG. 8, z eut GMO thin plates 11 and 1g (cut perpendicularly to the c axis) were bonded on both sides of a metal piece 1S with tlicirz` axes arranged in the same or the opposite direction thereby to form a bnnorph. One end 17 of the metal piece 15 was lixed while the other end was fixed with a cantilever element 19 having a sapphire needle 18 tixed in the free end thereof. Rubber dnmpers 16 were provided on both sides of the metal piece l5 on the plates l, and 13. When the pickaup thus constructed was moved along a groove 20, n bending moment was given to the bimorph. A polarization change was generated in GMO corresponding to the amount of bending, and a variation in electrical energy was generated on the surfaces of GMO plates 1l and 12.

(l) When the GMO plates are bonded on both sides of the metal piece 15 with their c axes arranged in the same direction as shown in FIG. 9, a voltage appeared across the electrodes 211 and 212. Since the bending of GMO plates 11 and 1.2 and hence their polarization changes were proportion@ to the amplitude of the groove,

' a voltage proportional to the amplitude of the groove could be obtained.

(2) When the z cut GMO crystals were bonded on both isides of the metal piece 1S with their.c axes arranged in the opposite direction and a pressure was given to the GMO plates 11 and 12, a voltage was generated in accordance with the principle as described in (l).

EMBODMENT 4 As shown in FIGS. 11(11) and (b), a 45 z cut GMO piece 1 (eut in parallel with the (001) and (110) surfaces being perpendicular to the c axis) with dimensions of 0.2 mm. in thickness, 20 mm. in length and 1.5 mm. in width was titted with two pairs of electrodes 2 and 2' on beth sides of the (O01) surface and a capacitor 2.2 of 0.1 at. on the side of the electrode 2'. W'hen an AC voltage of v. and 50 Hz. was applied to the primary terminals of electrode 21, a voltage was induced in the domain A of the GMO piece 1 and caused a variation in the spontaneous polarization. An electrical energy was generated in the secondary electrodes 2', yielding a voltage of 50 Hz. and 2 v. on the secondary terminal 23.

The ferroelectric-ferroelastie materials as used in the above embodiments can be employed in a mechano-electrical coupling device utilizing the polarization change of the crystals which preferably have the GMO crystal structure (wherein R and R' are at least one rare earth element, x=0-l.0 and e=00.2) belonging to the orthorhombic system and the point group mm2.

We claim:

1. A compound ferroelectric-ferroelastic crystal of GdzMooi) s having the gadolinium molybdate crystal structure belonging to the orthorhombic system and the point. group mm2 in which the spontaneous strain of the crystal has a value more than 10X10"3 said crystal having lattice constants of 0:10.388i0-005 A., b=l0.426:l: 0.005 A. and e== 10.7091101305 A.

2. A compound ferroelectric-ferroelnstie crystal of 81112010093 having the gadoliniunt molybdnte crystal structure belong to the orthorhombic system and the point groupmml in which the spontaneous strain otl the crystal has n value more than 1.0Xl0r3, said crystal having lattice constants of r11-10.4781111005 A., brljlli 0.005 A., and c==l0.SS6i0.005 A.

3. A compound ferroeleetric-ferroclnstle crystal of Eug(l\floO4l3 having the gadolinum molybdate crystal structure belonging to the orthoxhombic system and the point group mm2 in which the spontaneous strain of the crystalhas a value more than 10X10-3, said crystal having lattice constants of a=l0.377i0.005 A., b=10.472 0.005 A. and c=10.655i0.005 A.

4. A compound ferroelectrc-ferroelastic crystal of Dy3(MoO4)3 having the gadolium molybdate crystal structure belonging to the orthorhombic system and the point group mm2 in which the spontaneous strain of the crystal has a value more than 10X10-3, said crystal having lattice constants of a=10.331;l:0.005 A., b= 10.3361 0.005 A., and c== l0.603;*;0.005 A.

y .14 Rcfercnces Cited UNITED STATES PATENTS 3,437,432 4/1969 Borchardt 252-629 X 5 OSCAR R. VERTIZ, Primary Examiner J. COOPER, AAssistant Examiner U.S. Cl. X.R. 

